The first part of a two-parter (is this entire season actually going to be two-parters?), "The Zygon Invasion" acts as a follow-up of sorts to Doctor Who's 50th anniversary episode, 2013's "The Day of the Doctor". In it, we find out that since the human/Zygon treaty was created in "The Day of the Doctor" there has been an operation to allow Zygons to live on Earth by having them disguise themselves as humans. Unfortunately, being denied the right to be who they are, a small percentage of the Zygon population has become radicalised and seeks to take over the world.
As far as political commentaries go, it's hardly subtle. Written by Peter Harness (who also wrote last seasons equally political "Kill the Moon"), "The Zygon Invasion" is obvious allegory for the way that Great Britain has handled immigration in recent years, a topic more relevant than ever thanks to rising right-wing, nationalistic sentiment in the face of the European refugee crisis. The problem here comes from the fact that like in "Kill the Moon", Peter Harness seems to paying lip service to one side of the argument while the actual tissue of the episode supports the other, causing a disconnect that makes it near impossible to figure out which side of the fence the episode actually falls on.
That aside, "The Zygon Invasion" is a perfectly serviceable episode of Doctor Who, albeit one that seems to be pure set-up for a pay off that we won't see until next weeks "The Zygon Inversion". In a lot of ways it feels like last seasons "Dark Water"/"Death in Heaven" story, particularly the "Death in Heaven" half - we catch up with a couple of the characters that made an appearance in those episodes as well as spending a lot of time in different parts of the world, visual shorthand for "this episode has far reaching consequences". An easy way to create the feeling that the events of "The Zygon Invasion" are epic in the traditional sense of the word? Yes, of course - but one that works for the most part, splitting the action between London and New Mexico in order to demonstrate that the stakes are high.
The biggest feather in the cap of "The Zygon Invasion" is the episodes excellent sense of pacing though. The story progresses quickly but never feels rushed, and despite each of the different locations having their own story to deal with, the episode never ends up feeling like it is simply jumping between them randomly - it's well structured, something that often goes unappreciated in television for some reason. It's also worth mentioning how well "The Zygon Invasion" plays its big twist, refusing to give anything away until the perfect moment for the reveal presents itself and then making sure everyone understands how it happened to stop the twist from feeling unearned or cheap.
As is this case with any two-parter episode, we won't really know how good "The Zygon Invasion" is until we can see how the set-up undertaken here is paid off in next weeks "The Zygon Inversion", but I feel pretty optimistic about the direction it is heading in and I'm looking forward to seeing how certain plot points shown in "The Zygon Invasion" end up being resolved - particularly ones that might end up being related to Jenna Coleman's departure from the show. Either way, "The Zygon Invasion" is a fairly entertaining episode that fans of Doctor Who are sure to enjoy thanks to the way that it rewards those with a knowledge of episodes from earlier seasons without coming across as fan pandering - and any episode of Doctor Who that doesn't feel like it was made just to appease the often unbearable 'Whovians' is alright by me.